Backlight device for liquid crystal display

ABSTRACT

A backlight device for a liquid crystal display device includes a light source providing light; a light waveguide plate  40  guiding light from the light source, having an emitting surface, a front surface and a bottom surface, the emitting surface adjacent to the light source; a reflector arranged under the bottom surface of the light wave guide plate, reflecting light; and at least one cholesteric liquid crystal (CLC) film arranged over the front surface of the light wave guide plate, collimating light.

CROSS REFERENCE

This application claims the benefit of Korean Patent Application No.1999-38155, filed on Sep. 8, 1999, under 35 U.S.C. § 119, the entiretyof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight device, and moreparticularly, to a backlight device for use in a liquid crystal display(LCD) device.

2. Description of Related Art

FIG. 1 shows a conventional backlight device for use in an LCD device.As shown in FIG. 1, the conventional backlight device 100 includes alight source 10, a U-shaped lamp holder 15 having a lamp reflector 16, alight waveguide plate 40, a reflector 50, and a prism sheet 20. Thelight source 10 is located adjacent to an end 40 a of the lightwaveguide plate 40, and most of the light source 10 is surrounded by thelamp holder 15. The end 40 a constitutes a surface through which theincident light from the light source 10 is emitted. The prism sheet 20is arranged on a front surface 40 b of the light wave guide plate 40,and the reflector 50 is located under a bottom surface 40 c of the lightwave guide plate 40.

The emitted light from the light source 10 is directed toward thesurface 40 a of the light waveguide plate 40 by the lamp reflector 16and is then directed either toward a prism sheet 20 or a reflector 50.The light reflected by the reflector 50 is emitted toward the observerthrough a prism sheet 20, and a liquid crystal panel (not shown) in theform of an information.

FIG. 2 is a graph illustrating brightness with respect to an angle oflight passing through the prism sheet. As shown in FIG. 2, brightnessdistribution is discursive. In other words, light passing through theprism sheet 20 is not well collimated. As a result, light collimatingefficiency is bad, and collimating loss takes place at portions “A”. Inaddition, in the conventional backlight device of such a structure, itis not easy to narrow an angle of light passing through the prism sheetto a desirable angle range (for example, from −40 degrees to +40degrees).

For the foregoing reasons, there is a need for a backlight device havinga high light collimating efficiency.

SUMMARY OF THE INVENTION

To overcome the problem described above, preferred embodiments of thepresent invention provide a backlight device having a high lightcollimating efficiency.

In order to achieve the above object, a preferred embodiment of thepresent invention provides a backlight device for a liquid crystaldisplay device, including: a light source providing light; a lightwaveguide plate guiding light from the light source, having an emittingsurface, a front surface and a bottom surface, the emitting surfacebeing adjacent to the light source; a reflector arranged under thebottom surface of the light wave guide plate, reflecting light; and atleast one cholesteric liquid crystal (CLC) film arranged over the frontsurface of the light wave guide plate, collimating light.

The CLC film has a one-layered structure of one of either a right- or aleft handed CLC layer, wherein the right- and left handed CLC layerselectively reflect right- and left-handed circularly polarized light,respectively. The CLC film may also have a dual-layered structure of aright- and a left handed CLC layer, wherein the right- and left handedCLC layer selectively reflect right- and left-handed circularlypolarized light, respectively. The backlight device further includes aprism sheet arranged between the CLC film and the front surface of thelight wave guide plate.

By using the backlight device according to preferred embodiments of thepresent invention, light collimating efficiency and brightness can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which likereference numerals denote like parts, and in which:

FIG. 1 shows a conventional backlight device for use in a LCD device;

FIG. 2 is a graph illustrating brightness with respect to an angle oflight passing through a prism sheet;

FIG. 3 is a cross-sectional view illustrating a backlight deviceaccording to a preferred embodiment of the present invention;

FIG. 4 shows a principle that the CLC film 30 collimates light accordingto a preferred embodiment of the present invention;

FIG. 5A is a graph illustrating a reflectance with respect to awavelength of light vertically incident on a cholesteric liquid crystal(CLC) film according to a preferred embodiment of the present invention;

FIG. 5B is a graph illustrating a reflectance with respect to awavelength of light non-vertically incident on the CLC film;

FIG. 6 is a graph illustrating brightness with respect to an angle oflight passing through the CLC film;

FIGS. 7, 8 and 9 are cross-sectional views illustrating modifications ofthe backlight device according to a preferred embodiment of the presentinvention;

FIG. 10 is a graph illustrating brightness with respect to a wavelengthof spectrum; and

FIG. 11 is a cross sectional view illustrating another modification ofthe backlight device according to a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS

Reference will now be made in detail to preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 3 is a cross-sectional view illustrating a backlight deviceaccording to a preferred embodiment of the present invention. As shownin FIG. 3, the backlight device includes a light source 10, a U-shapedlamp holder 15 having a lamp reflector 16, a light waveguide plate 40, areflector 50, and a cholesteric liquid crystal (CLC) film 30. The lightsource 10 is located adjacent to an end 40 a of the light waveguideplate 40, and most parts of the light source 10 are surrounded by thelamp holder 15. The end 40 a constitutes a surface through which theincident light from the light source 10 is emitted. The CLC film 30 isarranged on a front surface 40 b of the light wave guide plate 40, andthe reflector 50 is located under a bottom surface 40 c of the lightwave guide plate 40.

The CLC film 30 serves to collimate light emitting from the light source10 and preferably has a dual-layered structure of right-handed andleft-handed CLC layers 30 a and 30 b. Since direction vector of theliquid crystal is helical in cholesteric state, the CLC film 30 reflectsonly light that is the same in helical director and satisfies Braggreflection condition. In other words, the right-handed CLC layer 30 areflects only a right-handed circularly polarized light, and theleft-handed CLC layer 30 b reflects only a left-handed circularlypolarized light. Using this principle, lost light can be restored,thereby improving the light collimating efficiency and brightness. TheCLC film 30 may have a one-layered structure of either the right- or theleft-handed CLC layer. However, since light has a right-handedcircularly polarized light and a left-handed circularly polarized light,it is preferable that the CLC film 30 has a dual-layered structure ofthe right- and the left-handed CLC layers 30 a and 30 b.

FIG. 4 shows a principle by which the CLC film 30 collimates light. Asshown in FIG. 3, a left-handed circularly polarized light R1 incidentwithin a certain angle, which has a wavelength less than 780 nm, passesthrough the left-handed CLC layer 30 b, and a left-handed polarizedlight R2 incident out of a certain angle, which has a wavelength lessthan 780 nm, is reflected. The left-handed polarized light R2 incidentout of a certain angle is reflected again by the light wave guide plate40 or a reflector (not shown) and directs toward the left-handed CLClayer 30 b. it is repeated until the left-handed polarized light R2incident out of a certain angle becomes light incident within a certainangle to pass through the CLC layer 30 b.

FIG. 5A is a graph illustrating a reflectance with respect to awavelength of light vertically incident on the CLC film. “R” denotes areflectance, and “λ₁” denotes a wavelength of vertically incident light.The CLC film 30 also serves to transmit only light of a certainwavelength by controlling a helical pitch. The wavelength of lightvertically incident on the CLC film 30 can be obtained by the followingformula: λ₀=P(n_(o)+n_(e))/2. In the formula, “λ₀” is a wave length ofvertically incident light, “P” is a helical pitch, “n_(o)” is anordinary refractive index, and “n_(e)” is an extraordinary refractiveindex. Therefore, by controlling a helical pitch of the CLC film 30, anideal visible ray having a wavelength of from 380 nm to 780 nm can betransmitted, as shown in FIG. 4. By controlling a helical pitch of theCLC film 30, it is also possible for the CLC film 30 to transmit awavelength of vertically incident light less than 780 nm in light of aprism sheet.

FIG. 5B is a graph illustrating a reflectance with respect to awavelength of light obliquely incident on the CLC film. “R” is areflectance, and “λ_(α)” is a wavelength of obliquely incident lightwith a certain angle “α”. A wavelength of obliquely incident light witha certain angle “α” can be obtained by the following formula:“λ_(α)”=λ₀cos[sin⁻¹{2 sinα/(n_(o)+n_(e))}]. In order to reflect avisible ray of from 380 nm to 780 nm and transmit a wavelength of lightgreater than 780 nm, it is preferable that an average refractive index“(n_(o)+n_(e))/2” of the CLC film 30 is as small as possible.

As described above, the CLC film 30 according to the preferredembodiment of the present invention selectively reflects light with awavelength of more than 780 nm. However, it is also possible toselectively reflect light with a wavelength of more than 600 nm bycontrolling a helical pitch of the CLC film 30 so that only a wavelengthof an ideal visible ray generated from the backlight device may betransmitted

FIG. 6 is a graph illustrating brightness with respect to an angle oflight passing through the CLC film 30. As shown in FIG. 6, by arrangingthe CLC film 30 having the right- and left-handed CLC layers 30 a and 30b, brightness distribution can be concentrated within a range between−40 degree and +40 degree.

The backlight device according to the preferred embodiment of thepresent invention may further include a prism sheet 20 as shown in FIG.7.

The backlight device according to the preferred embodiment of thepresent invention may further include a plurality of the CLC films 30without or with the prism sheet 20 as shown in FIGS. 8 and 9,respectively.

Further, one of the right- and left-handed CLC layers 30 a and 30 b maybe constructed to selectively reflect all the wavelength range of from400 nm to 1000 nm, and the other may be constructed to selectivelyreflect vertically incident light with a wavelength of more than 600 nm.As a result, since only one of the right- and the left-handed circularlypolarized lights is transmitted, a light collimating efficiency andbrightness can be improved. Further, it is possible to include the CLClayer which selectively reflects vertically incident light with awavelength of from 480 nm to 530 nm and selectively reflects verticallyincident light with a wavelength of from 560 nm to 610 nm in the lightof spectrum of the backlight device shown in FIG. 10. Therefore, a colorspot, which may occur when the plurality of CLC films are arranged sothat vertically incident light with a wavelength of more than 600 nm istransmitted, can be prevented, and the light collimating efficiency canbe improved to as narrow a range as between −30 degrees and +30 degrees.

FIG. 11 shows a modified backlight device according to another preferredembodiment of the present invention. As shown in FIG. 11, at least oneCLC film 30 may be arranged on the end 40 a of the light wave guideplate 40 adjacent to the light source 15. By doing this, light emittingfrom the light source 10 can be collimated. Therefore, when incidentlight within a certain angle (a range between about −30 degree and about+30 degree) is directed into the light wave guide plate 40, it is easyto design the light wave guide plate 40 to guide light in a desirabledistribution.

As described above, by using the backlight device according to preferredembodiments of the present invention, light collimating efficiency andbrightness can be improved.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A backlight device for a liquid crystal displaydevice, comprising: a light source providing light; a light waveguideplate guiding light from the light source, said light waveguide platehaving an emitting surface, a front surface and a bottom surface, theemitting surface being adjacent to the light source; a reflectorarranged under the bottom surface of the light waveguide plate,reflecting light; and at least one single layer cholesteric liquidcrystal (CLC) film arranged on the front surface of the light waveguideplate, selectively collimating light by controlling a helical pitch P ofsaid CLC film according to the equation: λ₀ =P(n _(o) +n _(e))/2, whereλ₀ is a wavelength of incident light, P is a helical pitch, n_(o) is anordinary refractive index, and n_(e) is an extraordinary refractiveindex.
 2. The backlight device of claim 1, wherein the single layer CLCfilm is one of either a right-handed or a left-handed CLC layer, theright-handed CLC layer selectively reflecting right-handed circularlypolarized light and the left-handed CLC layer reflecting left-handedcircularly polarized light.
 3. The backlight device of claim 1, whereinthe at least one single layer CLC film has a dual-layered structurehaving both a right-handed and a left-handed CLC layer, the right-handedand left-handed CLC layer selectively reflecting right-handed andleft-handed circularly polarized light, respectively.
 4. The backlightdevice of claim 1, further comprising a prism sheet arranged between theat least one single layer CLC film and the front surface of the lightwaveguide plate.
 5. A backlight device for a liquid crystal displaydevice, comprising: a light source providing light; a light waveguideplate guiding light from the light source, said light waveguide platehaving an emitting surface, a front surface and a bottom surface, theemitting surface being adjacent to the light source; a reflectorarranged under the bottom surface of the light waveguide plate,reflecting light; and at least one single layer cholesteric liquidcrystal (CLC) films arranged over the front surface of the lightwaveguide plate, collimating light, wherein the at least one CLC filmselectively reflects incident light with a wavelength of more than 600nm by controlling a helical pitch P of said CLC film according to theequation: λ₀ =P(n _(o) +n _(e))/2, where λ₀ is a wavelength ofvertically incident light, P is a helical pitch, n_(o) is an ordinaryrefractive index, and n_(e) is an extraordinary refractive index.
 6. Thebacklight device of claim 5, wherein each single layer CLC film is oneof either a right-handed or a left-handed CLC layer, each right-handedCLC layer selectively reflecting right-handed circularly polarized lightand each left-handed CLC layer reflecting left-handed circularlypolarized light.
 7. The backlight device of claim 5, wherein each singlelayer CLC film is formed by a dual-layered structure, each structurehaving both a right-handed and a left-handed CLC layer, the right-handedand left-handed CLC layers selectively reflecting right-handed andleft-handed circularly polarized light, respectively.
 8. The backlightdevice of claim 5, further comprising a prism sheet arranged between theat least one single layer CLC film and the front surface of the lightwaveguide plate.
 9. A backlight device for a liquid crystal displaydevice, comprising: a light source providing light; a light waveguideplate guiding light from the light source, said light waveguide platehaving an emitting surface, a front surface and a bottom surface, theemitting surface being adjacent to the light source, the length of saidemitting surface being substantially shorter than a length of the frontsurface; and at least one cholesteric liquid crystal (CLC) film arrangedon the emitting surface of the light waveguide plate, collimating lightby reflecting light outside the band of incedent light.
 10. Thebacklight device of claim 9, wherein the at least one CLC film is one ofeither a right-handed or a left-handed CLC layer, the right-handed CLClayer selectively reflecting right-handed circularly polarized light andthe left-handed CLC layer reflecting left-handed circularly polarizedlight.
 11. The backlight device of claim 9, wherein each of the at leastone CLC films is formed by a dual-layered structure, each structurehaving both a right-handed and a left-handed CLC layer, the right-handedand left-handed CLC layers selectively reflecting right-handed andleft-handed circularly polarized light, respectively.
 12. The backlightdevice of claim 9, further comprising a prism sheet arranged between theat least one CLC film and the front surface of the waveguide plate. 13.A backlight device for a liquid crystal display device, comprising: alight source providing light; a light waveguide plate guiding light fromthe light source, said light waveguide plate having an emitting surface,a front surface and a bottom surface, the emitting surface beingadjacent to the light source, the length of said emitting surface beingsubstantially shorter than a length of the front surface; a reflectorarranged under the bottom surface of the light waveguide plate,reflecting light; and at least one cholesteric liquid crystal (CLC) filmarranged on the emitting surface of the light waveguide plate adjacentto the light source reflecting light outside of the band of incedentlight.
 14. The backlight device of claim 13, wherein the single layerCLC film is one of either a right-handed or a left-handed CLC layer, theright-handed CLC layer selectively reflecting right-handed circularlypolarized light and the left-handed CLC layer reflecting left-handedcircularly polarized light.
 15. The backlight device of claim 13,wherein the at least one single layer CLC film has a dual-layeredstructure having both a right-handed and a left-handed CLC layer, theright-handed and left-handed CLC layer selectively reflecting right andleft-handed circularly polarized light, respectively.
 16. The backlightdevice of claim 13, further comprising a prism sheet arranged betweenthe at least one single layer CLC film and the front surface of thelight waveguide.